Regulation of Convection Onsets During the 15 December 1992 TOGA COARE mesoscale convective system

It is known that spatial and temporal variations in the convective heating profile over the warm pool affect significantly the planetary scale circulation in the tropics. Such variability may be introduced when mesoscale convective systems (MCS) evolve in time. Research into the lifecycle of MCSs over the warm pool is relatively rare because observations generally focus only on their mature and dissipating stages. This study attempts to fill this gap using a numerical modeling approach. Specifically, we simulated the 15 December 1992 MCS observed during TOGA-COARE (Tropical Ocean Global Atmosphere - Coupled Ocean-Atmosphere Response Experiment). Our objective is to determine the factors that control the predictability of the lifecycle of this MCS.

Our simulation reproduces realistically the growth and mature phases of the lifecycle of the MCS as verified against available observations. Using the technique proposed by Sherwood and Wahrlich, we classified the growth phase of the MCS into three convective onsets. The first and the second onsets occurred at 0700 and 1100 UTC respectively. They are responsible for the initiation and development of two cloud clusters denoted as S1(located at 2.5S 155E) and S2 (located 300-400 km east of S1). The third convective onset occurred at 1400 UTC and it resulted in the merger of S1 and S2. An extensive area with anvil cloud characterizes the mature stage of the simulated MCS (from 1700-2000 UTC). This result is consistent with airborne Doppler radar analysis which revealed the existence of two convective lines (aligned NE-SW at 4.5S, 156E) with their associated stratiform regions during the mature stage.

We found that the two main factors that determine the timing and location (or predictability) of deep convection are the large scale (grid scale) vertical motion and the surface potential temperature drop off (SPTD). The SPTD is defined as the difference between the potential temperature of the sea surface and the average potential temperature of the atmospheric boundary layer (ABL). Deep convection is initiated if the upward motion in the lowest 400 hPa is upward and the SPTD exceeds 1.2 C. The first convective onset (0700 UTC) is associated with a favorable SPTD and a large scale ascent in a transequatorial flow centered at 0N, 155E. The second convective onset (1100 UTC) occurs when the SPTD > 1.2 C for the first time. The third convective onset (1400 UTC) occurs when the vertical motion becomes upward in the lowest 400 hPa in association with the passage of the quasi-2 day wave. Comparison of the "MOIST" and "DRY" simulations demonstrated that the convective lines observed during the mature stage are caused by mesoscale circulation induced by deep convection in the region.